The present invention relates to substrate processing, and more specifically to a method and apparatus for forming a dielectric film such as a phosphosilicate glass (PSG) film having good gap fill capability, good stability, and compatibility with planarization techniques. The present invention is particularly useful when forming a dielectric film used for advanced premetal dielectric (PMD) layer applications. Of course, the dielectric film may also be useful for other applications.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two-year/half-size rule (often called "Moore's Law") which means that the number of devices that fit on a chip doubles every two years. Wafer fabrication plants today are routinely producing devices with 0.5 .mu.m and even 0.35 .mu.m size features. Fabrication plants soon will be producing devices having even smaller geometries. As device sizes become smaller and integration density increases, issues that were not previously considered crucial by the industry are becoming of paramount concern.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a film, such as a silicon oxide, on a semiconductor substrate. Silicon oxide is widely used as an insulating layer in the manufacture of semiconductor devices. As is well known, a silicon oxide film can be deposited by thermal chemical vapor deposition (CVD) or plasma-enhanced chemical vapor deposition (PECVD) processes. In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions (homogeneous or heterogeneous) take place to produce a desired film. In a conventional plasma process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film. Examples of apparatus utilizing conventional plasma processes include, for example, a capacitively-coupled parallel plate CVD apparatus, or an electron cyclotron resonance (ECR) CVD apparatus. In general, reaction rates in thermal and plasma processes may be controlled by controlling one or more of the following: temperature, pressure, and reactant gas flow rate.
One particular use for a silicon oxide film is as a separation layer between the polysilicon gate/interconnect layer and the first metal contact layer for MOS transistor connections. Such separation layers, referred to as PMD layers, are typically deposited before any of the metal layers in a multi-level metal structure. It is important that films used as PMD layers have low stress, good gettering capability, good gap fill capability, either good planarization characteristics or compatibility with planarization techniques, and low moisture absorption. Specifically, low stress in films used as PMD layers is required in order to avoid cracking or bowing of the wafer which can damage the devices formed thereon. In addition, films used as PMD layers need to have good gettering capability to trap mobile ions/charges (such as sodium or other metal ions) that may cause damage in the formed devices. When used as a PMD layer, the silicon oxide film is deposited over a lower level polysilicon gate/interconnect layer that usually contains raised or stepped surfaces, which create gaps that need to be adequately filled by the film. If these gaps are not adequately filled by the film, voids or seams in the film may occur and cause degraded device performance. The initially deposited film generally conforms to the topography of the poly layer and typically needs to be planarized before an overlying metal layer is deposited. A standard reflow process, in which the oxide film is heated to a temperature at which it flows, may be used to planarize the film. With small device dimensions, it is critical in some applications that reflow of PMD layers and other process steps be carried out below 800.degree. C. to maintain shallow junctions and prevent the degradation of self-aligned titanium silicide contact structures or the like. As an alternative to reflow, a chemical mechanical polishing (CMP) technique may be used to planarize the film. In addition, low moisture absorption is important for films used as PMD layers. Moisture absorbed into the film often reacts with the dopants in the film resulting in film crystallization, increasing the potential for undesirable cracking of the deposited film and leading to damaged devices.
Although typically used as PMD layers, borophosphosilicate glass (BPSG) films are becoming inadequate in some applications having tight thermal budgets. Because of their low stress, good gap fill capability, good gettering capability, and capability to reflow at high temperatures, BPSG films are an example of films that have been found particularly suitable for use as PMD layers. Standard BPSG films may be formed by introducing phosphorus and boron sources into a processing chamber along with the silicon and oxygen sources normally required to form a silicon oxide layer. Deposition techniques for such BPSG films include atmospheric pressure CVD (APCVD), sub-atmospheric pressure CVD (SACVD), low pressure CVD (LPCVD), and plasma-enhanced CVD (PECVD). Many semiconductor manufacturers utilize SiH.sub.4 -based BPSG processes that produce films requiring reflow at temperatures greater than about 900.degree. C. for about 0.5 .mu.m device geometries. At geometries less than 0.5 .mu.m, more stringent gap fill requirements may necessitate the use of chemistries such as tetraethylorthosilicate (TEOS) and ozone (O.sub.3) which provide films with excellent gap fill and reflow capability. For example, TEOS/O.sub.3 -based BPSG films produced by APCVD or SACVD achieve improved gap fill, but planarization by reflow at greater than about 850-900.degree. C. or by a rapid thermal process and a CMP step may be required for many applications. In general, typical BPSG films used for PMD applications may require a reflow process at temperatures greater than about 850.degree. C. and a CMP step in order to provide a planarized layer. However, such reflow temperatures are often too high for tighter thermal budgets that are increasingly required for advanced PMD applications in smaller geometry (0.25 .mu.m and less) devices. Further, other film properties such as low moisture absorption are important for films used as PMD layers. Typical BPSG films used as PMD layers may require certain boron concentrations in order to adequately reflow for planarization, but such films may be prone to increased moisture absorption at these boron concentrations. The absorption of moisture often reacts with the BPSG film resulting in film crystallization, which may lead to cracking of the deposited film to damage devices on the wafer.
As an alternative to BPSG films, PSG films deposited using PECVD or APCVD have been proposed but found inadequate for some advanced PMD applications. These PSG films tend to absorb moisture and undesirably have a high hydrogen and carbon content in the film. Further, such PSG films are often incompatible with CMP, especially for devices having higher aspect ratios. PECVD and APCVD techniques result in high conformality films which often leave seams in the middle of the gaps being filled. During CMP, these seams are easily attacked by the slurry, and these seams may even be observed by scanning electron microscopy (SEM).
From the above, it can be seen that other methods and apparatus for forming an oxide film having low stress, good gap fill capability, compatibility with planarization techniques, and low moisture absorption are needed for some advanced PMD applications.